1. Field of the Invention
This invention is directed to the chemical treatment of acid mine drainage to neutralize acid, remove metals, and remove sulfate ion without the formation of hydrous difficult-to-dewater iron hydroxide precipitate.
2. Description of Related Art
Acid mine drainage (AMD) is a major environmental problem where minerals and coal are mined and pyrite is present as an associated material. In the presence of oxygen and water, the pyrite oxidizes to produce sulfuric acid and soluble iron. The sulfuric acid reacts with other minerals in the associated ore, dissolving them and releasing additional heavy metal ions into solution. The resulting low pH solution containing these metals is an environmental hazard which is toxic to plants and animals.
AMD is a potential problem wherever minerals, coals, and other materials that contain pyrite are extracted from the earth. Pyrite, when exposed to air, oxidizes to produce ferrous ion, acid, and sulfate ion. The first step is shown in equation (1). EQU 2FeS.sub.2 (s)+2H.sub.2 O+7O.sub.2 =4H.sup.+ +4SO.sub.2.sup.2- +2Fe.sup.2+ (Eq. 1)
The next step is the oxidation of ferrous ion to ferric ion as shown in equation (2). EQU 4Fe.sup.2+ +O.sub.2 =4Fe.sup.3+ +2H.sub.2 O (Eq. 2)
At a pH below 3.5, the bacterium, Thiobacillus ferroxidans, often catalyzes the oxidation of iron. The ferric ion dissolves additional pyrite and provides a cycle for the dissolution of pyrite, see equation (3). EQU FeS.sub.2 (s)+14Fe.sup.3+ +8H.sub.2 O=15Fe.sup.2+ +2SO.sub.4.sup.2- +16H.sup.+ (Eq. 3)
The dissolved iron, sulfate and acid become part of the tailings pore water, and the acid begins to dissolve other metals in the tailings or associated rocks and soil. Eventually, the acidic water with dissolved ions drains from the tailings into ground water or surface water. The process results in water with a low pH (e.g. less than 3) that contains high levels of iron and sulfate with lesser amounts of various metal ions including aluminum, arsenic, cadmium, chromium, copper, lead, magnesium, manganese, mercury, silver, selenium, and zinc. Left untreated, Acid Mine Drainage can pollute large volumes of ground and surface water. In some cases, the drainage may continue over decades.
In one prior art process, AMD water is treated with bases such as lime or sodium hydroxide. The principal ions in solution are ferrous and ferric, both of which are acidic. Upon treatment with base to neutralize the pH, the ferrous and ferric ions begin to precipitate and produce an unsightly, very hydrous, amorphous, semigelatinous hydrated iron hydroxide precipitate, see equation (4). EQU Fe.sup..times.+ +2H.sub.2 O=Fe(OH).sub..times. (s) (Eq. 4)
where x=2 and 3.
Using this treatment procedure, large, undesirable, very hydrous, gelatinous flocs are produced that incorporate large quantities of water in their matrix making them difficult to handle and hard to dewater. Stoichiometric amounts of chemical are needed and low level contaminants may need a second stage treatment for complete removal.
Another chemical approach employs a zero valent metal such as scrap iron to precipitate the metals in solution through oxidation-reduction reactions. Other chemical or physical treatment approaches are ion exchange, activated alumina, and reverse osmosis. These approaches are often used as secondary steps to polish treatment streams where most of the contaminants have been removed.
Certain prior art biological treatments take advantage of sulfate reducing bacteria (SRB) to convert sulfate into sulfides. Metal sulfides are less soluble that the corresponding oxides and sulfide precipitation removes both from solution. The treated mixture needs a carbon source which is supplied by an organic waste material such as sewage.
In another similar prior art biological approach an artificial marsh is developed that consists of a series of ponds that are treated with compost, topsoil, and animal manure. Aquatic plants such as cattails are planted and crushed limestone is added to neutralize the acid. Cattails remove metals by adsorption, consumption and filtration. Algae and bacteria are able to grow easily in the marsh. Under reducing conditions, SRB reduces sulfate to sulfide and precipitates any metals in solution.
Biological treatments are used for low volume AMD generation started shortly after mining operations begin because of the large surface area requirements needed to establish and maintain the conditions required to successfully remove both metals and sulfates. Biological treatments are usually not suitable for large scale AMD projects that have been in operation for long periods of time.
The Resource Conservation and Recovery Act (RCRA), passed in 1976 and amended in 1984, classifies eight heavy metals as toxic. They are arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver. Hazardous wastes containing heavy metals are a major environmental problem because of their toxicity, persistence in the environment and potential mobility. Arsenic enters the environment from both natural and man-made sources, e.g. from natural sources such as the weathering of rocks, volcanoes, and as a by-product in the production of natural gas in some areas. Man made sources include herbicides; pesticides; smelting of zinc, copper and lead; fly ash from large scale burning of coal; tailings from mining wastes and use of industrial chemicals containing arsenic such as corrosion inhibitors.
Arsenic and its compounds are used in industry as pesticides, insecticides, and corrosion inhibitors. Many uses of arsenic have been discontinued because of environmental concerns about toxicity to both animals and humans. Nevertheless arsenic compounds are widely scattered throughout the environment. Continuing sources of arsenic atmospheric contamination are coal burning and copper smelting. The Environmental Protection Agency lists arsenic as a carcinogen. Federal and state regulations impose strict limits on arsenic concentrations in soil, air, and water. For example, the Toxic Substance Control Act (TSCA) has a reportable spill quantity of one pound for arsenic.
The effort to remove arsenic compounds from the environment includes efforts to remove it from antifreeze that protects the radiators of engines used with large compressors in the natural gas processing and pipeline industry. Antifreeze is a solution of glycols and water. One common glycol used in antifreeze is ethylene glycol. The amount of glycol used depends on the lowest anticipated winter temperature, typically in the range of forty to sixty percent glycol by volume. Since arsenic compounds have been used as corrosion inhibitors, many such industrial cooling systems are contaminated with arsenic.
If new antifreeze and new corrosion inhibitors are used, old solutions with arsenic in them must be disposed of in an environmentally safe manner. Burning arsenic contaminated antifreeze may increase air pollution in the same manner as burning coal with arsenic contaminated ash. The continued use of contaminated antifreeze poses a human health risk problem in the event of spills and worker exposure. An antifreeze spill of 1200 gallons containing 100 mg per liter arsenic is a reportable spill. The reportable spill quantity for ethylene glycol is 5000 pounds or 9000 gallons as antifreeze. The arsenic content of contaminated antifreeze may range to 500 mg per liter. The level of heavy metals may be up to 10 ppm. Volumes of contaminated antifreeze may range up to over 50,000 gallons at a single location.
Certain prior art processes are used to remove the build-up of minerals and metals (e.g. barium, cadmium, lead, chromium, copper) from used antifreeze solutions. The general processes involve use reverse osmosis technology; ultra-filtration technology; treatment with molecular sieves; treatment with ion exchange resins; and treatment with activated carbon. These processes may remove arsenic at low concentrations (e.g. at 10 to 20 ppm) but the treatment becomes uneconomical at high arsenic concentrations (e.g. at 25 ppm) because of the low efficiency and poor specificity. Certain prior art chemical treatment processes remove arsenic from antifreeze. One of these employs polyacrylate (a co-polymer of acrylic acid and acrylamide) and ethylene diamine tetra-acetic acid (EDTA) as its major components. Another process uses a formulation with sodium nitrite and potassium hydroxide as its major components. In one prior art process iron sulfate or alum is used to remove arsenic from wastewater. In certain nonanalogous prior art methods heavy metals are removed from water by increasing pH by adding a base (e.g. calcium hydroxide or magnesium hydroxide) until the metals precipitate.